1. Field of the Invention
The present invention relates generally to a method and apparatus for removing particulate contaminants from liquid in a radioactive environment and more particularly to concentrating the filtered radioactive suspended solids for storage. The method and apparatus contemplate flowing nuclear reactor liquid contaminated with low conductivity radioactive waste first through a fine mesh pre-filter for removal of the larger contaminants and then through a deep-bed filter for removal of the very fine particulates.
2. The Prior Art
Filtration of particulate contaminants from liquid used in nuclear power plants, such as pressure water reactors or boiling water reactors, has posed numerous problems in the prior art. Quite naturally, some of these problems are a direct result of the radioactive nature of the liquid and particulate contaminants. Another reason for the prior art problems results from the wide range in size of the particulate contaminant which can appear within the circulating liquid in the nuclear power plant. For example, the contaminanats range in size from very fine iron oxides and oxide products that are virtually non-filterable to resin beads used in ion exchange filters. Between these two extremes, other particulate contaminants such as lint, floor and laundry wastes, powdered resin used in ion exchange filters, and filter aid materials such as diatomaceous earth or fiber cellulose filter media may also be found in the circulating liquid of the nuclear power plant.
Various attempts have been made in the prior art to effectively remove the various-sized particulate contaminants, but these prior art attempts have exhibited shortcomings in their inability to remove the very fine contaminants or in their high cost of manufacture and operation.
One such approach in the prior art was to separate all the different types of waste into separate containers for separate filtration and removal of each type contaminants. This approach has proved unsatisfactory in some cases due to the inability to properly remove the contaminants to an acceptable level and in substantially all cases due to the cost involved in providing separate filtration assemblies for each type of contaminant.
Another prior art approach has been to use large clarifiers to settle the heavier suspended contaminant solids and then filter the supernate through pre-coat and body feed filters. This particular approach has proved to be unsatisfactory because it is necessary to use pre-coat material such as diatomaceous earth or cellulose fiber that becomes radioactively contaminated and requires storing. An additional disadvantage of this particular type of prior art approach is the necessity in adding body feed to the accumulated contaminants to maintain sufficient porosity so that filtration can continue for a reasonable period of time. The body feed material added to such a unit also becomes radioactively contaminated and likewise requires storing. It has turned out in many cases with this type of filtration that there is up to ten times as much filter-aid material required as there is original radioactive contaminants. Thus, the amount of contaminated solids that require storing are multiplied ten times with this type filter arrangement.
An example of a prior art filtration process employing both the two previously described filtration assemblies with the inherent shortcomings is disclosed in U.S. Pat. No. 3,773,177.
Centrifugation is a third example of a prior art attempt to remove particulate contaminants from liquid in a nuclear power plant. The primary problem with this type of separator is the inability to remove the very fine particulate contaminants and contaminants having a very light specific gravity. Another problem is considerable wear on the centrifuge due to the gritty materials present in the reactor liquids.
The present invention overcomes these prior art problems of concentrating the contaminants to a small, easily storable volume since it is not necessary to add large dosages of filtering aid materials. More importantly, the present invention achieves a clarified effluent capable of meeting standards of almost any nuclear power plant in the world, so that the clarified liquid may be recirculated for further use.
Definition and Explanation of Terms
For purposes of simplifying designations of filter media sizes, all references herein to a particular "mesh" or "mesh size" refer to standard U.S. Sieve Series (also known as U.S. Standard Mesh Sieve size). A designation of a mesh size preceded by a minus (-) sign indicates that all granules will pass that size sieve; or on the average are finer than the sieve size when determining an average size of filter medium. A designation of mesh size preceded by a plus(+) sign indicates that all granules will be retained on that size sieve; or on the average are coarser than that sieve size when determining an average size of filter medium. For example, a layer or bed of granular filter media designated as -8 to +30 mesh or between -8 and 30 mesh means all the granules will pass a No. 8 U.S. Sieve Series mesh and all the granules will be retained on a No. 30 U.S. Sieve Series mesh. Stated another way, all the granules are smaller than a No. 8 sieve and larger than a No. 30 sieve. A medium designated as having an average size of between -8 and +30 mesh means that the average sized filter granule in the medium will pass a No. 8 U.S. Sieve Series mesh and be retained on a No. 30 U.S. Sieve Series mesh.
"Average size" of filter medium is defined as a mathematically derived figure equal to the sum of the individual products of the fraction by weight of each mesh size in a layer of medium multiplied by the respective mesh sizes. For example, in a filter bed consisting of a layer of granulated black walnut shells, having 40% by weight of 6 mesh granules and 60% by weight of 8 mesh granules, and a layer of silica sand, having 50% by weight of 20 mesh and 50% by weight of 30 mesh, the "average size" of filter medium in the respective layers is 7.2 mesh (0.40 .times. mesh + 0.60 .times. 8 mesh = 7.2 mesh) and 25.0 mesh (0.50 .times. 20 mesh + 0.50 .times. 30 mesh = 25.0 mesh).